CN109073827B

CN109073827B - Optical monitoring mechanism, external resonator laser light source, wavelength tunable laser device, and optical waveguide filter

Info

Publication number: CN109073827B
Application number: CN201780024660.5A
Authority: CN
Inventors: 小林直树
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 2016-04-21
Filing date: 2017-03-30
Publication date: 2020-08-28
Anticipated expiration: 2037-03-30
Also published as: JPWO2017183411A1; JP6544484B2; WO2017183411A1; CN109073827A; US20190131768A1

Abstract

An optical monitoring mechanism for monitoring light in an optical circuit (10) comprising: a loop-back shaped loop-back mirror (12), a linear optical waveguide (11) connected to the loop-back mirror (12), the optical monitoring mechanism having a structure in which a loop-back shaped or loop-back shaped tap port (15) is placed close to a position on the loop-back mirror (12) apart from a connection point between the loop-back mirror (12) and the optical waveguide (11) at a position where optical lengths are equal when light travels clockwise and when light travels counterclockwise, which makes it possible to extract a part of light from the loop-back mirror (12) to the tap port (15) as monitoring light without optical loss. Thus, an optical monitoring mechanism having a structure that minimizes the occurrence of optical loss when light for monitoring is extracted is provided.

Description

Optical monitoring mechanism, external resonator laser light source, wavelength tunable laser device, and optical waveguide filter

Technical Field

The present invention relates to an optical monitoring mechanism, an external resonator type laser light source, a wavelength tunable laser device, and an optical waveguide filter, and particularly relates to an optical monitoring mechanism, an external resonator type laser light source, a wavelength tunable laser device, and an optical waveguide filter that realize tapping (lossless tapping) with reduced loss.

Background

A digital coherent communication system is a communication system that can transmit a large amount of information by performing wavelength multiplexing in one optical fiber. A wavelength tunable laser device used in an existing digital coherent communication system is provided with two light sources as tunable local oscillator light sources, one light source for transmission and the other light source for reception. Each light source is equipped with a light monitoring mechanism for monitoring the wavelength, output intensity, S/N ratio (signal-to-noise ratio), etc. of emitted light.

Further, it is being considered to install only one light source instead of two light sources on a future digital coherent communication system to achieve downsizing and low power consumption. However, since the number of light sources is reduced from two to one, the one light source needs to be operated with high output, and further improvement in monitoring accuracy is required.

An example of an external resonator type laser light source including a semiconductor optical amplifier and a waveguide type optical circuit is described hereinafter by way of illustration. Fig. 3A and 3B are structural diagrams illustrating an exemplary external resonator type laser light source having a simple structure without a light monitoring mechanism. Fig. 3A shows the structure of an external resonator type laser light source. Fig. 3B is a view showing an optical path and a traveling direction in the external resonator type laser light source having the structure shown in fig. 3A by arrows. In the external resonator type laser light source, an external filter and a Semiconductor Optical Amplifier (SOA) are spatially combined by using a lens to form a resonator structure. With this resonator structure, a high-performance filter can be used, and a relatively wide wavelength tunable range is realized.

As shown in fig. 3A, the external resonator type laser light source includes an optical circuit 10A and a semiconductor optical amplifier 20. The optical circuit 10A includes an optical waveguide 11 and a loop-back mirror 12 in a loop-back shape. The loop back mirror 12 acts as a ring resonator and its round trip length is determined by the wavelength of the incoming light. As shown in fig. 3B, the light travels in the semiconductor optical amplifier 20 and the optical circuit 10A, and is output as an amplified light output.

Specifically, light emitted in the forward path by applying a current to the semiconductor optical amplifier 20 is output from the semiconductor optical amplifier 20 to the optical circuit 10A. Thereafter, as shown in fig. 3B, the light from the semiconductor optical amplifier 20 travels through the optical waveguide 11 in the optical path 10A, and then branches in both clockwise and counterclockwise directions at the connection point with the loop back mirror 12. Each of the lights branched in the two directions travels through the loopback mirror 12 and loops back to the optical waveguide 11 through the loopback structure. The semiconductor optical amplifier 20 functions as an amplifier for light traveling in one direction toward the light output side. The semiconductor optical amplifier 20 is coated with an antireflection coating on its end face. The light in the return path is amplified by the semiconductor optical amplifier 20 as it is directed towards the end face. The light looped back in the return path through the loop back mirror 12 is input to the semiconductor optical amplifier 20 and amplified by the semiconductor optical amplifier 20, and then, a part of the light passes through the end face and is output to the outside as light output. A part of the light amplified by the semiconductor optical amplifier 20 in the return path is reflected by the end face, becomes light in the forward path, and enters the loop back mirror 12 through the optical waveguide 11. In this way, in the external resonator type laser light source in fig. 3, the light in the return path that has been looped back through the optical waveguide 11 is amplified by the semiconductor optical amplifier 20, and a part of the light in the return path is reflected by the end face on the light output side, and returns again after repeatedly traveling through the optical waveguide 11 and the loop-back mirror 12. This structure having the semiconductor optical amplifier 20, the optical waveguide 11, and the loop back mirror 12 enables laser light to be emitted and a part of the laser light to be output as output light from the end face to the outside.

An external resonator type laser light source having a structure according to the related art equipped with a light monitoring mechanism is described below. Fig. 4A and 4B are structural diagrams illustrating an exemplary external resonator type laser light source according to the related art equipped with a light monitoring mechanism. Fig. 4A illustrates a structure of an external resonator type laser light source, and fig. 4B is a view illustrating an optical path and a traveling direction in the external resonator type laser light source having the structure illustrated in fig. 4A by arrows.

As shown in fig. 4A, the optical circuit 10B further includes a tap port 17 (i.e., an optical waveguide for monitoring) as an optical tap for monitoring the wavelength, intensity, and the like of light (see patent documents 1 and 2), and this optical circuit 10B is different from the optical circuit 10A in fig. 3A. Note that optical tap (optical tap) refers to an operation of extracting a part of light or extracting light. The tap port 17 is formed in a curved shape, and a part of the tap port 17 is close to the optical waveguide 11 from the semiconductor optical amplifier 20 to the loopback mirror 12 to branch a part of the light in the return path and extract a part (about several percent) of the light in the return path.

Therefore, as shown in fig. 4B, a part of the light in the return path toward the loop-back mirror 12 is extracted to the tap port 17 (optical waveguide for monitoring), and is output as monitoring output light through the optical waveguide on the monitor port 18 side.

Reference list

Patent document

PTL 1: japanese laid-open patent publication No. 2015-212687

PTL 2: international patent publication No. WO2013/114578

Disclosure of Invention

Technical problem

However, according to the related art, when a part of light is extracted for monitoring in an optical monitoring mechanism for monitoring the wavelength, output intensity, S/N ratio (signal-to-noise ratio), and the like of the light, excessive loss occurs. It is important to reduce this loss. To achieve high output operation, it is desirable that the light loss depletion in the light source may be small. In particular, in a structure having one light source instead of two light sources, in order to downsize, it is necessary to reduce the light loss to a level as small as possible in the light monitoring mechanism to achieve a high output operation.

However, according to the related art, the tap port 17 having the structure as shown in fig. 4 is used as an optical monitoring mechanism, and a part of the light in the return path looped back from the loop-back mirror 12 and traveling toward the semiconductor optical amplifier 20 through the optical waveguide 11 is also extracted to the tap port 17, then travels toward the optical waveguide on the side of the dump port 19 in the direction opposite to the monitor port 18, and is discarded as unintended monitoring light (i.e., ineffective light), which causes optical loss to occur. Therefore, the problem to be solved in the optical apparatus including the optical monitoring mechanism according to the related art shown in fig. 4 is: unnecessary loss of light is minimized in light monitoring.

(object of the invention)

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an optical monitoring mechanism, an external resonator type laser light source, a wavelength tunable laser device, and an optical waveguide filter having a structure that minimizes the occurrence of optical loss when light for monitoring is extracted.

Technical scheme

In order to solve the above-described problems, the optical monitoring mechanism, the external resonator type laser light source, the wavelength tunable laser device, and the optical waveguide filter according to the present invention mainly have the following characteristic structures.

(1) The optical monitoring mechanism according to the present invention includes: a loop-back mirror of a loop-back shape to which the optical waveguide is connected; and a loop-back-shaped or loop-shaped tap port placed near a position on the loop-back mirror where optical lengths are equal when the light travels clockwise and when the light travels counterclockwise from a connection point between the loop-back mirror and the optical waveguide, wherein a part of the light from the loop-back mirror is extracted to the tap port as monitoring light.

(2) The external resonator type laser light source according to the present invention is an external resonator type laser light source including a semiconductor optical amplifier and a waveguide type optical circuit, wherein the optical monitoring mechanism described in (1) above is used as an optical monitoring mechanism for monitoring light in the optical circuit.

(3) The wavelength tunable laser device according to the present invention is a wavelength tunable laser device that emits laser light having a tunable wavelength, in which the external resonator type laser light source described in (2) above is used as a light source that emits laser light.

(4) The optical waveguide filter according to the present invention is an optical waveguide filter for separating wavelength-multiplexed light by wavelength using an optical waveguide filter, wherein the optical monitoring mechanism described in (1) above is used as an optical monitoring mechanism for monitoring the wavelength-multiplexed light input to the optical waveguide filter.

Advantageous effects

The optical monitoring mechanism, the external resonator type laser light source, the wavelength tunable laser device, and the optical waveguide filter according to the present invention have the following advantageous effects.

Specifically, according to the present invention, since the loop back shape or the loop shape of the tap structure is adopted as the optical monitoring means for monitoring the intensity, wavelength, S/N ratio, and the like of light, it is possible to extract the monitoring light without causing unnecessary optical loss.

Drawings

FIG. 1A is a block diagram illustrating an exemplary optical monitoring mechanism according to the present invention.

FIG. 1B is a block diagram illustrating an exemplary optical monitoring mechanism according to the present invention.

Fig. 2 is a structural diagram showing an exemplary structure of an optical waveguide filter according to the present invention.

Fig. 3A is a structural view showing an exemplary external resonator type laser light source having a simple structure without a light monitoring mechanism.

Fig. 3B is a structural view showing an exemplary external resonator type laser light source having a simple structure without a light monitoring mechanism.

Fig. 4A is a structural view showing an exemplary external resonator type laser light source according to the related art having a light monitoring mechanism.

Fig. 4B is a structural view showing an exemplary external resonator type laser light source according to the related art having a light monitoring mechanism.

Detailed Description

Preferred embodiments of an optical monitoring mechanism, an external resonator type laser light source, a wavelength tunable laser device, and an optical waveguide filter according to the present invention are described below with reference to the accompanying drawings. Note that although the optical monitoring mechanism and the optical waveguide filter according to the present invention are described in the following description, an external resonator type laser light source including a semiconductor optical amplifier and a waveguide type optical circuit or a wavelength tunable laser device that emits laser light having a tunable wavelength used in a digital coherent communication system may of course have a structure with such an optical monitoring mechanism. Further, the reference numerals in each of the drawings described below are shown by way of illustration only for easier understanding, and are not intended to limit the present invention to those components illustrated in the drawings.

(features of the invention)

Before describing embodiments of the present invention, features of the present invention will be summarized first. The main feature of the present invention is a tap structure having a loop back shape or a loop shape as an optical monitoring mechanism for monitoring the intensity, wavelength, S/N ratio, and the like of light. Specifically, the main feature is to place a loop back shape or a loop back shaped optical tap near a loop back mirror unit in an optical circuit, thereby realizing an optical monitoring mechanism having a structure capable of monitoring optical power with reduced optical loss.

More specifically, the main feature is to employ a light monitoring mechanism having the following structure.

In particular, it is characterized in that: along with the loop-back mirror including the loop-back shaped optical waveguide, an optical tap including a linear optical waveguide for guiding guided light to an open end of the optical loop and the loop-back shaped or loop-shaped optical waveguide (tap port) is placed on the loop-back mirror at a position distant from one end of the loop-back mirror to which the optical waveguide for inputting the guided light to and outputting the guided light from the loop-back mirror is connected, at which an optical length is equal when the light travels clockwise around the loop and when the light travels counterclockwise around the loop. Further, regarding the optical tap, an end of the optical waveguide (tap port) to which the linear optical waveguide for guiding the guided light to the open end of the optical circuit is connected, which is distant from the loop-back shaped or loop-shaped optical waveguide (tap port), is placed closest to the loop-back mirror at a position where the optical length is equal when the light travels clockwise around the loop and when the light travels counterclockwise around the loop.

This eliminates the need to place an unnecessary opening (dump port) in the optical circuit as an optical monitoring mechanism for monitoring the wavelength, output intensity, S/N ratio, and the like of light, so that the occurrence of excessive optical loss can be suppressed.

Such an optical monitoring mechanism can be suitably applied to an external resonator type laser light source composed of an optical waveguide having a loop back mirror and a semiconductor optical amplifier, a tunable filter, and the like. For example, in an external resonator type laser light source, by placing an optical monitoring mechanism according to the present invention (i.e., an optical tap structure in a loop shape (or a loopback shape)) and a loopback mirror in a waveguide type optical circuit, a light source in which optical loss does not occur when monitoring light is extracted can be realized.

(according to the embodiment of the invention)

An embodiment of a light monitoring mechanism according to the present invention is described below with reference to fig. 1. Fig. 1A and 1B are structural diagrams showing an exemplary optical monitoring mechanism according to the present invention, fig. 1A showing an example of the structure of the optical monitoring mechanism, and fig. 1B being a view showing an optical path and a traveling direction in the optical monitoring mechanism in fig. 1A by arrows.

As shown in fig. 1A, a feature of the optical monitoring mechanism according to the present invention is to place a loop shape or a loop shape optical tap close to a loop mirror having a loop structure. As shown in fig. 4A, the optical monitoring mechanism is placed in an optical device comprising an optical circuit 10 and a semiconductor optical amplifier 20. The optical circuit 10 has a structure including a linear optical waveguide 11, a loop-back mirror 12 in a loop shape, and further including a tap port 15 in a loop shape (or a loop-back shape) as an optical tap for optical monitoring and a linear optical waveguide 16. Optical tapping refers to an operation of extracting a part of light or extracting light. The optical tap for optical monitoring, which includes a loop-shaped (or loop-back shaped) tap port 15 and a linear-shaped optical waveguide 16, is different from the optical monitoring mechanism according to the related art shown in fig. 4. In the optical monitoring mechanism according to the related art shown in fig. 4, a portion of the tap port 17 of the curved shape near the optical waveguide 11 is used as an optical tap for optical monitoring.

The round trip length (optical length) of the loop-back mirror 12 composed of a ring-shaped optical waveguide is determined by the wavelength of the input light, and it functions as a ring resonator. The tap port 15 is a loop-shaped or loop-back shaped optical waveguide whose round trip length (optical length) is the same as that of the loop-back mirror 12 on the monitored side, and which is connected to a linear optical waveguide 16 that guides guided light to the open end 14 of the optical loop 10.

A tap port 15 is placed in the loop 10 near the loop back mirror 12 on the side being monitored. The position of the tap port 15 placed close to the loop-back mirror 12 is a position that is distant from the connection position of the optical waveguide 11 for inputting guided light to the loop-back mirror 12 and outputting guided light from the loop-back mirror 12, the optical length of which is equal when light travels clockwise around the loop-back mirror 12 and when light travels counterclockwise around the loop-back mirror 12 (i.e., a position on the loop-back mirror 12 opposite to the connection position of the optical waveguide 11).

Further, the tap port 15 is placed so as to be closest to the loop-back mirror 12 on the monitored side from a position (i.e., a position on the tap port 15 opposite to the connection position of the optical waveguide 16) at which the connection position of the optical waveguide 16 is equal in optical length when the light travels clockwise around the tap port 15 and when the light travels counterclockwise around the tap port 15. In other words, the linear optical waveguide 16 is connected to a position on the tap port 15 that is located at a distance from the loop-back mirror 12 on the tap port 15 closest to the side to be monitored, the optical length being equal when the light travels clockwise and when the light travels counterclockwise. The optical monitoring mechanism in which the loop-back mirror 12, the tap port 15, and the optical waveguide 16 are optically connected in this way can combine the monitoring light traveling clockwise around the tap port 15 and the monitoring light traveling counterclockwise around the tap port 15 at the connection point on the tap port 15 to which the optical waveguide 16 is connected, and extract the light to the outside through the optical waveguide 16 and the open end 14.

By using the optical monitoring mechanism whose tap structure is in the loop shape or the loop back shape as described above, it is not necessary to place an emptying port, so that, for example, the output intensity, wavelength, and the like of laser light from an external resonator type laser light source composed of a semiconductor optical amplifier and a waveguide type optical loop having a loop back mirror can be monitored without occurrence of optical loss.

(description of operation of the embodiment)

An example of the operation of the optical monitoring mechanism shown in fig. 1A is described with reference to fig. 1B.

As shown in fig. 1B, light to be monitored travels in the semiconductor optical amplifier 20 and the optical circuit 10A, and is output as an amplified optical output. Specifically, the light in the forward path travels through the semiconductor optical amplifier 20 and is input to the optical circuit 10.

Thereafter, as shown in fig. 1B, the light from the semiconductor optical amplifier 20 travels through the optical waveguide 11 in the optical loop 10, and then branches in two directions (clockwise and counterclockwise) at the connection point with the loop back mirror 12. Each beam of light travels through the loop-back mirror 12, passes through a position opposite to the connection position of the optical waveguide 11 (connection position with the tap port 15), and returns to the connection point with the optical waveguide 11. The two returned lights are combined at the connection point with the optical waveguide 11 and looped back to the optical waveguide 11.

At this time, a part of both the clockwise light and the counterclockwise light is extracted as the monitoring light to the tap port 15, and the tap port 15 is placed close to a position on the loop-back mirror 12 opposite to the connection position of the optical waveguide 11 (i.e., a position where the optical length is equal when the light travels clockwise around the loop-back mirror 12 and when the light travels counterclockwise). Specifically, a part of the light that has traveled clockwise around the loop-back mirror 12 is extracted as the monitoring light traveling counterclockwise around the loop-back shaped tap port 15, and a part of the light traveling counterclockwise around the loop-back mirror 12 is extracted as the monitoring light traveling clockwise around the loop-back shaped tap port 15.

On the other hand, the light in the return path that has looped back to the optical waveguide 11 is again input to the semiconductor optical amplifier 20, amplified and output to the outside as an optical output. Note that, in the semiconductor optical amplifier 20, a part of the light in the return path is reflected by the end face on the light output side, and returns again after repeatedly traveling through the optical waveguide 11 and the loop back mirror 12 to emit laser light, and the light is output as amplified light output from the semiconductor optical amplifier 20 to the outside.

On the other hand, each of the two monitoring lights extracted to the tap port 15, i.e., the monitoring light traveling in both the clockwise direction and the counterclockwise direction, travels around the tap port 15 of the loop-back shape to a position opposite to the position on the tap port 15 of the loop-back mirror 12 closest to the monitored side (i.e., a position where the optical length is equal when the light travels clockwise around the tap port 15 and when the light travels counterclockwise around the tap port 15). At the relative position, the two monitoring lights are combined, travel through the optical waveguide 16 connected to the relative position, are output as an optical output for monitoring from the open end 14 of the optical circuit 10, and are further output to the outside from the waveguide substrate on which the monitoring mechanism is mounted.

The two monitoring lights are combined at the connection point of the tap port 15 and the optical waveguide 16, and because they are combined at a position distant from the position where a part of the light extracted by the loop-back mirror 12 on the monitored side as the monitoring light is equal in optical length when the light travels clockwise around the tap port 15 and when the light travels counterclockwise around the tap port 15, the two monitoring lights have exactly the same light intensity and phase, and therefore, they are combined without occurrence of optical loss. Note that since optical loss occurs if the light intensity and phase of the two monitoring lights differ at the position where these lights are combined, it is preferable that the position and direction of placing the tap port 15 are as accurate as possible. Further, it is important that, in each of the optical waveguides of the loop-back shaped tap port 15 and the loop-back mirror 12, optical characteristics including an optical loss ratio and an optical phase shift ratio are identical between the optical waveguides in the clockwise direction and the counterclockwise direction.

Since the optical monitoring mechanism in which the configuration of the tap port 15 is the loop back shape or the loop shape as described above is employed, the dump port 19 as shown in fig. 4 is no longer placed, and optical loss does not occur when the monitoring light is extracted. It is thereby possible to monitor the output intensity, wavelength, etc. of laser light from the external resonator type laser light source composed of the semiconductor optical amplifier 20 and the waveguide type optical circuit 10 having the loopback mirror 12 without optical loss. Note that if the tap port structure has a shape other than the loop-back shape described above, the intensity of the monitoring light differs depending on the traveling direction of the monitoring light, which causes a loss to occur at the position where the lights merge.

(description of effects of the embodiments)

As described in detail above, since the loop back shape or the loop shape of the tap structure is adopted as the optical monitoring mechanism for monitoring the intensity, wavelength, S/N ratio, and the like of light in the present embodiment, the monitoring light can be extracted without unnecessary optical loss.

(embodiments of optical waveguide Filter)

An embodiment of an optical waveguide filter having a mechanism for extracting guided light according to wavelength is described hereinafter with reference to fig. 2. Fig. 2 is a structural diagram showing an example of the structure of an optical waveguide filter according to the present invention, and it shows an example of the structure of an optical waveguide filter including an optical tunable filter for separating only a specific tunable wavelength and extracting the specific tunable wavelength as an optical wavelength filtering mechanism for separating wavelength-multiplexed light by wavelength.

In optical waveguide filters, it has become increasingly important to have a mechanism for monitoring and controlling the state (wavelength, intensity, S/N ratio, etc.) of an optical signal. Therefore, there is a promising trend to adopt the structure of the optical waveguide filter 30 having the optical monitoring mechanism as shown in fig. 2. Note that although an optical waveguide filter composed of an optical tunable filter as an optical wavelength filter is described in this example, the present invention is not limited to this case, and an optical fixed wavelength filter may of course be used.

The optical waveguide filter 30 shown in fig. 2 has a structure including two stages of optical

tunable filters

32A and 32B as an optical wavelength filtering mechanism 31, and further includes an optical waveguide 11 and a loop back mirror 12. Further, as an optical monitoring mechanism, a loop shape to which the linear optical waveguide 16 is connected or a loop-back shaped tap port 15 is placed close to a position on the loop-back mirror 12 opposite to the connection point of the optical waveguide 11 (i.e., a position distant from the connection point of the optical waveguide 11 at which the optical length is equal when the light travels clockwise around the loop-back mirror 12 and when the light travels counterclockwise around the loop-back mirror 12) in the structure. In other words, the optical waveguide filter 30 has an optical wavelength filter mechanism 31 that separates wavelength-multiplexed light by wavelength, and employs the optical monitoring mechanism shown in fig. 1 as the optical monitoring mechanism.

While the invention has been particularly shown and described with reference to embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

The present application is based on and claims the benefit of priority from Japanese patent application No. 2016-085209, filed 2016, 4, 21, 2016, the disclosure of which is incorporated herein by reference in its entirety.

List of reference numerals

10 optical circuit

10A optical circuit

10B optical circuit

11 optical waveguide

12 LOOPBACK MIRROR (LOOPBACK MIRROR)

14 open end

15 TAP PORT (TAP PORT)

16 optical waveguide

17 tap port

18 monitor port

19 emptying PORT (DUMP PORT)

20 semiconductor optical amplifier

30 optical waveguide filter

31 optical wavelength filtering mechanism

32A TUNABLE FILTER (Optical TUNABLE FILTER)

32B TUNABLE FILTER (Optical TUNABLE FILTER)

Claims

1. A light monitoring mechanism, comprising:

a first optical waveguide;

a loop back mirror in a loop back shape, the loop back mirror having a connection point, the connection point connected to the first optical waveguide; and

a loop back shaped or loop back shaped tap port positioned proximate to the loop back mirror at a location on the loop back mirror equidistant from the connection point between the loop back mirror and the first optical waveguide that is equal in optical length when light travels clockwise and when light travels counterclockwise,

wherein a portion of the light from the loopback mirror is extracted to the tap port as monitoring light.

2. The optical monitoring mechanism of claim 1, wherein

A second optical waveguide is connected to another connection point on the tap port that is equal in optical length when light travels clockwise and when light travels counterclockwise from a position on the tap port that is closest to the loop-back mirror, and

the monitoring light traveling clockwise around the drop port and the monitoring light traveling counterclockwise around the drop port are combined at a connection point on the drop port to which the second optical waveguide is connected, and extracted to the outside through the optical waveguide as an optical output for monitoring.

3. Optical monitoring mechanism according to claim 1 or 2, wherein

The optical characteristics including the optical loss rate and the optical phase shift rate of the optical waveguide proceeding clockwise around the loop-back mirror from the connection point with the optical waveguide are the same as the optical characteristics including the optical loss rate and the optical phase shift rate of the optical waveguide proceeding counterclockwise around the loop-back mirror, and

the optical characteristics including the optical loss ratio and the optical phase shift ratio of the optical waveguide traveling clockwise from the position on the tap port closest to the loop-back mirror are the same as those of the optical waveguide traveling counterclockwise.

4. An external resonator type laser light source comprising a semiconductor optical amplifier and a waveguide type optical circuit, wherein the optical monitoring mechanism according to claim 1 or 2 is used as an optical monitoring mechanism for monitoring light in the optical circuit.

5. A wavelength tunable laser device that emits laser light having a tunable wavelength, wherein the external resonator type laser light source according to claim 4 is used as a light source that emits the laser light.

6. An optical waveguide filter that separates wavelength-multiplexed light by an optical wavelength filter according to wavelength, wherein the optical monitoring mechanism according to claim 1 or 2 is used as an optical monitoring mechanism for monitoring the wavelength-multiplexed light input to the optical waveguide filter.